Experts Question Usefulness of Air Force’s Robotic X-37B Space Plane

by Jeremy Hsu, SPACE.com Senior Writer | November 29, 2010 02:42pm ET

The U.S. Air Force's X-37B (shown here in an illustration) is an unpiloted military space plane capable of long-duration flights in Earth orbit. The robotic spacecraft has a solar array for power and a payload bay the size of a pickup truck bed. Credit: USAF

New observations of the Air Force's
secretive unmanned X-37B
space plane have put the spacecraft's classified mission back in the
limelight.
While many ponder the exact nature of such a vehicle, some experts have
already
gone on record questioning whether a robotic space plane is even needed.

Cheaper alternatives exist for just
about every mission
capability that the X-37B
robotic space plane might possess, according to an analysis
by the Union of
Concerned Scientists (UCS) released during the spacecraft's current
mission.
The organization's experts want the Obama administration and Congress
to review
the U.S. commitment to a space plane program.

Perhaps the main unique advantage of
the X-37B is the
ability to return experiments or hardware to Earth for inspection, as
Air Force
officials pointed out before the space plane's launch in April. Only
NASA's
space shuttle currently has that capability, but the shuttle is slated
to fly its
final missions during the next year or so.

Much of the uncertainty also rests
with the program's
unknown price tag. The space
plane's long history includes not just Boeing Phantom Works
as primary
contractor for the U.S. Air Force, but also NASA and the Pentagon's
Defense Advanced
Research Projects Agency (DARPA).

Counting the ways

The main issue with the X-37B is a
lack of a compelling
mission, according to Grego and UCS colleague David Wright, who
described their
concerns in June on their UCS blog "All Things Nuclear." They also
recently co-wrote "Securing the Skies" ? a report that outlines 10
steps the U.S. should take to safeguard space.

Other systems that don't return to
Earth can carry payloads
into orbit, maneuver in space, meet up with satellites, and release
multiple
satellites or other payloads at much lower cost, Grego and Wright said.

The ability to bring a payload back
to Earth represents a
fairly unique capability
for the X-37B. Yet the UCS researchers said satellite repair
or recovery
don't make much sense because of the X-37B's small payload bay, which
is just
the size of a pickup truck bed.

By comparison, the space shuttle's
payload bay could fit two
entire X-37Bs inside.

As for deploying space marines, the
experts say forget about
it. An X-37B's gliding approach for landing would make it easy prey for
air
defense systems, and hardly justify the high cost of dropping off a few
fighters.

"It's hard to think of what could
make that mission
compelling," Grego told SPACE.com. "It doesn't protect you from
antiaircraft
fire, and the element of surprise doesn't really work in your favor if
you're
launching on Atlas V [rocket]."

Similarly, the X-37B represents a
very unlikely carrier of missiles
or other weapons because of its payload restrictions, costs
and vulnerability
as a slow, gliding bomber, according to the UCS researchers.

Their assessment largely agrees with
an earlier analysis by
Brian Weeden, a former U.S. Air Force orbital analyst and now technical
adviser
for the Secure World Foundation. Weeden had rated the likelihood of a
weaponized X-37B as "zero."

The orbital spy

Weeden agreed with the UCS
researchers about reusable space
planes having a "host of engineering and technical issues" that limit
the cost-effectiveness of such vehicles. But he still sees the X-37B as
a reasonably
capable orbital spy that can fill gaps in current coverage by
orbital
satellites

"I would say that right now the space
plane is probably
more responsive in terms of meeting a war fighter [operationally
responsive
space] need than anything else we have," Weeden said. "But that
depends a lot on the turnaround time."

An X-37B could reconfigure its sensor
payload for each mission.
That payload would depend upon the needs of a military commander or
intelligence agency to peek at certain parts of the world, Weeden
suggested.

Having more propellant to burn per
mission than a permanent
satellite could also enable the X-37B to change its orbit, and
therefore have
the flexibility to change its coverage area on the fly.

Finally, the space plane has a
possible time advantage over
a satellite launch mission in terms of preparations, Weeden said. Each
satellite design has a unique way of "mating" with the booster rocket
that carries it into orbit, which can become a time issue on short
notice.

Air Force officials might bypass that
issue during an
emergency by simply sticking whatever sensor payload into the standard
body
of the X-37B.

Without knowing the exact costs
involved (they're
classified), much of the debate about the X-37B might also come down to
viewpoints. Where the UCS researchers and even Weeden see a lack of any
single
compelling mission, the Air Force and the U.S. Department of Defense
may savor
the realm of possibilities.

Weeden noted the Department of
Defense has often favored
such jack-of-all-trades systems during recent years.

"The X-37B doesn't do one thing
really well, but it
does a lot of things," Weeden explained. "So in that sense, I can see
the argument for it."

Robotic re-entry

Much of the speculation about the
X-37B's reusability ultimately
depends upon how well it performs during re-entry and landing. Such an
automated plane-like landing would represent a first for the U.S. Air
Force.

Only the Soviet Union has previously
demonstrated such an
automated gliding return capability with its Buran version of the U.S.
space
shuttle.

"From everything I know about it,
it's not that much
more advanced than the shuttle," Weeden said. "It's evolutionary, not
revolutionary, with respect to the shuttle."

Such a landing certainly has
advantages for delicate or
secretive military and intelligence payloads compared to a capsule
parachute
landing.

"It's nicer to have it land on your
runway than having
to scoop it up in the ocean or from a landing in the desert," Grego
admitted. Yet she and Wright at the Union of Concerned Scientists
remain
unconvinced that the costs justify the space vehicle.

Whatever the X-37B's capabilities
now, it started out as a
fairly ambitious NASA project under the X-37 designation. NASA and
Boeing
signed a $173 million cooperative agreement to jointly develop the
vehicle back
in 1999.

The Air Force took early interest in
the X-37 as a possible
space maneuvering vehicle, according to Susan Turner, former deputy
manager for
NASA's X-37 program. But the U.S. space agency had more interest in
advancing
the overall technologies of reusable
space vehicles.

Building a better space plane

The space agency aimed to include a
whole new batch of
technologies, Turner said. Those included the automated re-entry and
landing
system.

NASA also wanted flight controls
driven entirely by
electromechanical actuators (EMA), which eliminate the bulky hydraulic
circuits
in normal control systems and take the approach known as power-by-wire.
The X-37's
braking system would have also incorporated EMA.

New thermal tiles were required to
protect the X-37 against
the fiery heat of re-entry, because of its steeper inclination approach
compared with the space shuttle. The heat load also rises because of
the X-37's
smaller size, which means less surface area to bleed off heat.

So-called hot structures were planned
for the ruddervators,
which combine the elevators and rudder control surfaces that help steer
an
aerospace vehicle. The hot structures aimed to shrug off the re-entry
heat load
without requiring additional tiles or blankets for protection.

"We were going to fill the sky with
X-planes,"
Turner recalled. She is currently with the engineering directorate at
NASA's
Marshall Spaceflight Center in Huntsville, Ala.

But the X-37 only managed to survive
as a NASA project until
2004, when President George Bush announced his moon-centered NASA
initiative
that left no room for a reusable mini-shuttle vehicle.

Eventually, NASA transferred the X-37
project to the
Pentagon's DARPA, which in turn handed off the project to the Air Force.

X-37 reborn

Size restrictions prevent the X-37B
from becoming a
full-fledged shuttle replacement, at least as far as flying humans is
concerned.
In fact, NASA had once considered flying the predecessor X-37 as a
secondary
payload within the shuttle, Turner said.

"[X-37] was always simply a
technology demonstrator
that didn't put people at risk, and cost less in theory than the
shuttle,"
Turner said. She added that NASA also hoped at one point to use the
X-37 as a
long-duration orbital vehicle, which is not unlike how the Air Force
may
consider the X-37B.

The X-37 could have also launched
more often than the
shuttle aboard an expendable Atlas or Delta 4 medium-class rocket. Such
missions could have taken science payloads into orbit more frequently
rather
than get in the long line for a shuttle launch, and still take
advantage of X-37's
return capability.

Similarly, NASA had interest in the
X-37 as a reusable
platform to fly hardware in space for testing purposes, so that
engineers could
know how well such systems fared. Those testing opportunities come
rarely,
Turner said.

Cost and effort aside, Turner seemed
glad the X-37 finally
reached orbit as the X-37B, even as outside experts continue to debate
its
purpose and cost-effectiveness as an Air Force vehicle.